When i open Unity it appears a blue background and i cant do anything. I want to solve that.
place some objcts. some lights, move the main camera so it can render some objects in the scene 
Thats the problem i cant place anything
if you get the blue screen when you press play, it’s only natural, since you have no objects placed in the environment. To do that you must first enable Scene View, to do this you must click on the small icon, that looks like 3 horizontal lines, on the top right of any unity window and select Add Tab and select Scene. and you will have a scene view in which you will add objects.
After you’ve done the things above, go to GameObject->Create Other and select Cube, after that press F. you should center the Scene view on the cube.
I had the same problem, I opened a project but it didn’t work until I ALSO opened a scene. File->Open Scene or File->New Scene